Yellowstone County is a county located in the State of Montana. As of the 2012 United States Census Bureau Estimate put its population at 151,882;[1] it is the most populous county in Montana. Its county seat is Billings. Yellowstone County is named for the Yellowstone River which roughly bisects the county from southwest to northeast. The river in turn was named after the yellow Sandstone cliffs in what is now Yellowstone county.[2]